A credential establishes one as a reliable source or worker? in some context.

professional credentials

Normally a degree such as a PhD or MBA? establishes a set of credentials.

In medicine and other fields of very high risk, credentialing? is a process in itself requiring a strict protocol before a person is allowed, for instance, to operate on someone in an operating room.

negative credentials

There are also negative credentials that establish lack of reliability or safety. However in politics as usual, often one credential serves both functions. For instance if someone is PNG in a group, opponents may make them very welcome. While a PhD in a subject might imply that a person is an academic and not suited for practical decision making - "overqualified". Even a criminal record? is not always a negative credential - for instance in some cases the ability to forge? or crack a safe might be very desired, e.g. in military intelligence? work. Very few credentials are near-universally negative with the possible exception of PHB, HIV-positive?.

blind credentials

It is often the case that credentials are presented without any identity information. That is, some standard of evidence is met that proves that someone "is a member" or "has paid the fee to enter" but which does not identify them specifically, or isn't used to identify them specifically (like a loyalty card? used in a coffee shop to make the tenth cup free, or a stamp on the hand).

The most sophisticated form of blind credential? is based on cryptography? methods, and is used to create digital cash?.

alleged credentials

Often there is statistical evidence? or hearsay? or rumour? involved in a credential, especially a negative one which people might seek to avoid.